Mobile radio involves communication between mobile units and a base station via transmission of radio signals. One method of communication is known as Time Division Multiple Access (TDMA) in which a period of time is divided into multiple timeslots, each of which is assigned to a different communicating unit. In order for a receiving unit to decode the signals intended for it, the receiving unit must synchronize to the timeslot assigned to it. Therefore, each message sent to a receiving unit has a synchronization signal included therein. Once the receiving unit is synchronized to the sending unit, the receiving unit can decode the symbols being sent.
Unfortunately, the transmitted signal may be reflected off obstructions (such as trees, buildings, etc.) which are in the path between the sending and receiving units. The obstructions cause a portion or portions of the transmitted signal to arrive at the receiving unit after the main portion is received. This is known as multipath propagation and must be compensated within the receiving unit. If the mobile station is moving, the signal it receives can also be subject to fading.
Furthermore, the local oscillators which provide the timing and frequency basis in the mobile and base stations often drift apart. Of particular concern is that of frequency drift, or offset. The frequency offset increases the error probability in the receiving unit. In addition, it is typically desired that the mobile station lock onto the received frequency for transmission purposes, to enable the base station to decode the transmission of the mobile units without having to perform frequency acquisition.
U.S. Pat. No. 5,177,740 to Toy et al. describes a synchronization method which estimates the synchronization timing but has no correction for frequency offsets.
U.S. Pat. No. 5,343,498 to Toy et al. describes one synchronization system and method which initially estimates the timing of the synchronization words after which it estimates the frequency drift. The frequency offset is then corrected at the frequency of the local oscillator.